The present invention relates to a process for preparing a sintered body of aluminum nitride and more particularly, to a process for preparing aluminum nitride which uses an organoyttrium compound as a sintering aid and dispersant.
Aluminum nitride (AlN) possesses high thermal conductivity, electrical resistivity and a coefficient of thermal expansion compatible to silicon. In addition, AlN is a high strength ceramic material capable of maintaining its high strength characteristics even at high temperatures and extreme thermal shock. Because of these properties, AlN is used as a heat resistant material for high temperature applications and as a heat dissipating material for substrates of semiconductor devices.
An AlN sintered body is usually prepared by forming the ceramic powder into the desired shape and sintering at high temperatures. However, because of its highly covalent structure, it is very difficult to sinter AlN powders to full density without the addition of a sintering aid. Alkaline-earth metal compounds as well as rare-earth metal compounds are known as sintering aids for AlN. These compounds can react with oxides contained in the powder to form a liquid phase, which promotes particle rearrangement and grain growth as a result of a solution-precipitation mechanism. K. Komeya et al., in "Effects of Various Additives on Sintering of Aluminum Nitride", Translation of the Journal of the Ceramic Society of Japan, Vol. 89, No. 6, 1981, pp. 330-36, discloses that the addition of 5% by weight of Y.sub.2 O.sub.3 substantially improved the densification of AlN and attributes this improved densification to the formation of a nitrogen containing aluminate liquid in the system aluminum nitride-yttrium oxide.
The use of yttrium compounds as sintering aids for AlN is described in several publications. Shinozaki, et al., in "Sintering Behavior and Thermal Characteristics of Pressureless Sintered AlN with Y.sub.2 O.sub.3 Addition", 2nd Symposium on Basic Science of Ceramics, Yogyo-Kyokai, January 1984, found that the addition of at least 0.5% by weight of Y.sub.2 O.sub.3 not only enables good densification, but also makes it possible to form a product having a higher thermal conductivity than a product obtained without adding any such aid.
U.S. Pat. No. 3,833,389 issued Sep. 3, 1974, to Komeya et al., teaches that instead of using oxides of lanthanum, cerium, scandium, yttrium and yttrium aluminum garnet, or mixtures thereof, a material which is capable of producing one of the oxides under the sintering conditions can be used. For example, instead of using yttrium oxide, yttrium carbonyl (Y.sub.2 OC) or yttrium hydroxide (Y(OH).sub.3) may be used to form yttrium oxide prior to sintering.
U.S. Pat. No. 4,578,232 issued Mar. 25, 1986, to Huseby et al., discloses that the sintering aid, yttrium oxide, can be provided by means of a yttrium oxide precursor. The term yttrium oxide precursor means any organic or inorganic compound which decomposes completely at a temperature below about 1200.degree. C. to form yttrium oxide and a by-product gas which vaporizes away leaving no contaminants in the sintered body which would be detrimental to the thermal conductivity. Examples of these precursors are yttrium acetate, yttrium carbonate, yttrium oxalate, yttrium nitrate, yttrium sulfate and yttrium hydroxide.
The thermal conductivity of aluminum nitride is a function of its dissolved oxygen content and decreases with an increase in dissolved oxygen. Because aluminum nitride powder has an affinity for oxygen, especially when its surface is not covered by an oxide, it is necessary to remove the excess oxygen when high thermal conductivity is important. Aqueous solutions for the processing of AlN powder are not desirable since the powder will react with the water to liberate NH.sub.3 and form an Al.sub.2 O.sub.3 precursor such as AlOOH or Al(OH).sub.3, which increases the oxygen content of the powder. The sintering aids of the present invention are compatible with non-aqueous solutions and thus help to prevent the addition of excess oxygen to the AlN powder during processing.
A major problem with the use of sintering aids is the ability to uniformly distribute the aid prior to sintering. Most sintering aids are added as separate powders with the hope that mixing the powders will result in a uniform distribution. A need exists, therefore, for a process of uniformly distributing the sintering aid to obtain a sintered body having uniform chemical and electrical properties, in addition to high mechanical strength and high thermal conductivity.